The present invention relates to a DA converter testing system for testing a DA converter for converting digital information into analog information and, more particularly, to a DA converter testing system suitable for testing the dynamic characteristics of a DA converter.
Recently, demand for the DA converter capable of rapid conversion of a digital signal into an analog signal has expanded sharply in the fields of high-resolution video graphic displays and OA equipments, and the importance of methods and apparatus for testing the dynamic characteristics, such as glitches and settling time, of the DA converter affecting the performance of the DA converter during the transition of input digital codes has increased accordingly. A measuring method of measuring the linearity of the DA converter is disclosed, for example, in Japanese Unexamined Patent Publication No. 58-172562.
FIG. 14 is a block diagram showing the constitution of the above-mentioned testing system, and FIG. 15 is a timing diagram of assistance in explaining the relation between the conversion clock signal and the converted output. A conversion clock generated by a clock generator 200 regulates the speed of conversion of digital data supplied to a test DA converter 3. The start and stop of the conversion clock generated by the clock generator 200 are controlled by control signals provided by a control unit 110. A counter 30 counts the conversion clocks and gives signals having a digital code for sequentially increasing the DA-converted outputs to the test DA converter 3. The converted output of the test DA converter 3 is converted according to a conversion command having a speed equal to the conversion clock speed into a digital signal by an AD converter 4. When the input digital code applied to the test DA converter 3 and the level of an expected DA-converted output are different from each other, the output digital code of the AD converter 4 and the input digital code for the test DA converter 3 are different from each other. Accordingly, the conversion characteristics of the test DA converter 3 can be known through the comparison of the input digital code and the output digital code.
However, the above-mentioned conventional system has the following problems. That is, the conventional system is unable to test significant test items, namely, fast phenomena such as spikes (so-called glitches) that appears in the output waveform during the transition of the input digital code, and settling time, among the dynamic characteristics of the DA converter. In the conventional system, glitches superposed on the outut waveform during the transition of the input digital code given to the test DA converter is applied just as they are to the test DA converter. When such fast spike waveforms are applied to the AD converter, the effective resolution of the AD converter is deteriorated, the conversion action is unable to follow the input, the accuracy of the test is deteriorated, and large errors are caused in the test results. Furthermore, even if the conversion action is able to follow the input, the accurate AD conversion of the fast phenomena which are far faster than that of ordinary glitches is impossible, because the test DA converter and the AD converter are operating at the same conversion speed.